Q.Why is PCl5 a known compound, but NCl3 is not?

Phosphorus forms both trihalides ($PX_3$) and pentahalides ($PX_5$) because it can use empty 3d orbitals to expand its octet, unlike nitrogen, which is restricted to a maximum covalence of 4 and so cannot form $NCl_5$. Pentahalides are more covalent than the corresponding trihalides of the same Group 15 element because the higher oxidation state on the same small central atom pulls electron density more strongly, increasing polarisation and covalent character (Fajans' rules). $PCl_3$ hydrolyses readily and fumes in moist air because it reacts vigorously with atmospheric moisture, $PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl$, releasing HCl gas as visible white fumes. In $PCl_5$, which has a trigonal bipyramidal shape, the three equatorial P-Cl bonds are shorter and stronger than the two longer, weaker axial bonds (which experience greater repulsion from the three equatorial bonds), so all …